A fiber laser is usually composed of a pump source, a gain fiber, and a resonant cavity. The gain fiber is the key component of the fiber laser, which is generally the rare-earth-ion-doped glass fiber. The pump source provides energy for spontaneous emission, stimulated emission, and amplification from the rare earth ions in the gain fiber. The resonant cavity realizes round-trip oscillation of the signal light and finally obtains the laser output. The fiber lasers are composed of gain fiber and optical fiber devices, such as fiber grating, fiber coupler, fiber polarization controller, and so on, which are fusion spliced or connected to each other. Compared with gas lasers and traditional solid-state lasers, the fiber lasers have advantages of compact structure, high beam quality, good heat dissipation, high stability, and tunable operating wavelength, which have been widely used in military and civilian fields such as optical fiber communication, biomedical, material processing, atmospheric monitoring, laser radar, and so on. With the development of technology and the needs of society, many applications today require light sources to realize high-power, stable, low-noise, high-efficiency, and tunable laser output over a wide wavelength range. Therefore, ultra-wideband tunable, high-power, and ultra-compact fiber lasers have become a research hotspot in recent years. In addition, the operating wavelength of the fiber laser continues to extend in a mid-infrared region. For example, the 2.0 μm laser is located in a human eye safety waveband and is in a low-loss window of atmospheric transmission, and covers spectral absorption peaks of molecules such as H2O, CO, CO2, etc., which has important application value in many fields such as laser radar, laser detection, laser medical treatment, environmental monitoring, and so on. Therefore, the output of the near-mid-infrared tunable laser has aroused the research upsurge of domestic and foreign scholars.
Currently, a tuning range of mainstream tunable lasers is typically limited to a single waveband. More recently, researchers have proposed an ultra-wideband, continuously tunable, mid-infrared fiber laser that includes three different rare-earth-ion-doped optical fibers, three corresponding laser pump sources, and a planar diffraction grating. By connecting three kinds of optical fibers doped with different rare earth ions in parallel, and using a same planar diffraction grating as a wavelength tuning device, a continuously tunable laser output in a wavelength range from 2.8 μm to 4 μm was realized. Although the above solution can realize the tunable mid-infrared fiber laser output, it adopts a parallel branch method, which requires a plurality of pump sources of different wavelengths, multiple gain fibers doped with different rare earth ions, and a large number of passive components, leading to large loss, complicated preparation process, and uncompact structure, which is not conducive to realizing the low threshold, high energy transformation efficiency, and ultra-compact tunable fiber laser.
In addition to building a new cavity structure fiber laser, gain fibers with new components and new structures have also become research hotspots. Due to a limited emission wavelength of a single rare earth ion luminescent center doped optical fibers, the researchers introduce different rare earth luminescent ions to a glass optical fiber by co-doping, which can be used to realize the wideband tunable laser output by utilizing the emission wavelengths of different rare earth ions. Compared with the glass optical fiber doped with a single rare earth ion, the way by co-doping a plurality of rare earth luminescence centers can increase a width of an emission spectrum to a certain extent, but the increment is limited. At the same time, there are large non-radiative transitions between different luminescent rare earth ions, and a large amount of thermal load is caused, thereby reducing luminous efficiency and laser performance. More innovatively, there is proposed a multi-doped rare earth ions multi-core double-cladding optical fiber in which a plurality of cores of independent rare earth ions are disposed in a core. Different emission wavelengths are generated by using different rare earth ions in a single gain fiber, which can realize signal output in a wide wavelength range. However, a utilization rate of the core during pumping is low, and other wavebands of fluorescence emission are generated, thereby generating extra heat, reducing laser performance, and failing to achieve high efficient wideband tunable laser output.
In addition, from the perspective of the pumping mechanism, the entire core of the gain fiber is generally pumped simultaneously. Even in the above-described multi-core optical fiber, the pump light was coupled into the entire optical fiber. This method of full-core pumping inevitably simultaneously excites all rare earth ions in the core. Therefore, except for the laser at the resonant wavelength, the fluorescence of the remaining wavebands will eventually be converted into a large amount of heat, inevitably introducing the noises and reducing the stability of the fiber laser. On the other hand, different rare earth ions need to use the pump sources with different wavelengths. For example, Tm3+ ions generally use a semiconductor laser of 808 nm as the pump source, while Er3+ ions generally use a semiconductor laser of 980 nm as the pump source. The lasers with multiple wavelengths are required as the pump sources in multi-doped rare earth ions, resulting in an uncompact structure and complex operation of the laser.
It can be seen that the tunable laser output with compact structure, high efficiency, high power and low noise cannot be realized at present, and in particular, wideband and ultra-wideband near-mid-infrared tunable laser output cannot be realized. How to selectively and efficiently utilize a variety of rare earth luminescent ions in a single optical fiber to achieve a dual effects of the high luminous efficiency and the wide emission spectrum, and to avoid the generation of excess heat during pumping, and how to solve the problem of the complex operation and the uncompact structure due to the pump sources with multiple wavelengths required in a single optical fiber doped with different rare earth ions, are urgent problems to realize high performance wideband tunable laser output.